Add basic utility module romannumerals

As LilyPond variable names can't contain numbers it is common to use roman numerals to index names. Therefore it seems useful to add support for this to an editor like Frescobaldi.

In its initial state this module contains the conversion routines
- int_to_roman and
- roman_to_int
ly/util.py: Add range check for int2roman

roman numerals must be positive integers, but the current implementation
of int2roman processed negative numbers without complaining.

Now the function immediately returns an empty string if input < 1.
Not sure if raising an exception would be better.
Update romannumerals/__init__.py

remove int_to_roman (is defined in ly/util.py)
clean up roman_to_int (roman2int)

Add helper functions
- is_roman_letter()
- is_roman_numeral()
romannumerals: Add romansuffix2int

Function to return an integer corresponding to a
roman numeral suffix to a variable name

variableXII -> 12

This will be useful for batch processing variables